Titanium dioxide (TiO2) is a ubiquitous white pigment used in the paint and coatings industry, and is also prevalent in the semiconductor industry. TiO2 exists both naturally and synthetically in three forms: rutile, anatase, and brookite. Synthetic methods to prepare TiO2 typically involve variants of hydrolysis of titanium tetrachloride (TiCl4) or titanium oxychloride (titanyl chloride). For example, it has been known for over 100 years that reacting TiCl4 with water results in TiO2 by the following reaction (see B. J. Harrington, Trans. Royal Soc. (Canada), [2], 1, 3 (1895)):TiCl4+2H2O→TiO2+4HClAs is readily observed, HCl is a by-product of such hydrolyses. Such an acidic environment can also be problematic in many applications. For example, such an acidic environment can break down the binders and other additives to materials having incorporated TiO2, or react with substrates to which a TiO2— containing coating or material is applied. It should also be noted that TiCl4 is a hazardous material, mainly due to the acid-byproducts caused by rapid hydrolysis, and it requires special handling precautions.
As noted in the Encyclopedia of Chemical Reactions, vol. 7, page 404 “[r]utile crystals are obtained by the action of water vapor upon volatile titanium chloride.” The above reaction has been used by the TiO2 producing industries to produce bulk TiO2 powders in large quantities. As used herein, bulk powder means a powder having an average particle size of greater than 100 nm, such as 1 micron or greater.
For a wide range of commercial applications, materials with one or more of the following properties are desirable: (a) the ability to form nanoparticles which can be dispersed in both water as well as organic solvents, (b) a high optical transparency in the visible range (400-700 nm) and high UV absorption (wavelength below 400 nm), (c) maintaining the optical properties described in (b) above, while increasing particle loading density in other materials beyond just a few weight percentage, such as beyond 5-10 weight percent, and (d) absence of a shell of different material on the nanoparticles to allow the nanoparticles to link or chemically bond with solid matrix materials, such as polymers. Early transition metal-based sol gels (i.e. sols), such as those of Ti, Zr, or Hf, may exhibit such desirable properties.
Hence, preparation of titanium and zirconium sols are desired in which residual acid and metal oxide formation due to hydrolysis is minimized and the optical and electrical properties of the materials are preserved.